Generally, a communication between terminals located in a close distance may be defined as peer-to-peer configuration. In the peer-to-peer communication, a random access scheme between communication subjects is defined and the communication subjects perform communications according to a prescribed protocol. And, in the peer-to-peer communication, it may be unnecessary to consider whether a communication subject is connected to a public internet network.
On the other hand, it is mandatory for a communication in a cellular network to be defined as a communication between a base station or an existence equivalent to the base station and a terminal. All communication behaviors are controlled by the base station or the existence equivalent to the base station. In particular, the cellular network is configured to obtain a maximum throughput by put limitation on operations of all terminals according to a predetermined rule. Yet, this rule may be inefficient in accordance with an application or a channel environment of a terminal.
For instance, when a channel environment of a terminal is vulnerable, limitation is put on using an optimal communication path by finding a new access path. FIG. 1 is a diagram for a case of performing a communication via a neighbor terminal when a channel environment of a terminal is vulnerable.
Referring to FIG. 1, when a terminal is located in a shadow area, if the terminal performs a communication with a base station via a neighbor terminal having a good channel environment, it may be able to improve power efficiency and throughput of the terminal. However, in a cellular network of a related art, since it is mandatory for a communication to be performed via a base station, it may be difficult to obtain such an advantage as shown in FIG. 1.
According to the related art, terminals should perform short-range communications via a base station, each of the terminals consumes considerable amount of power. In particular, in case that a communication is performed between terminals in short distance, it is under the control of a base station and accompanies with such a considerably unreasonable communication structure as data is forwarded to a base station and then transmitted to a target terminal.
A cellular network does not allow a terminal-to-terminal communication but supports a similar structure. For instance, there is a data transmitting structure in which a terminal transmits data by accessing a useful relay node nearby instead of directly communicating with a base station. If the terminal uses the relay node, it may be able to obtain higher spectral efficiency with smaller power than forwarding data to the base station in direct.
Yet, a relay node is not a terminal but works as an access point for connecting to a network, which is a considerable disadvantage. In particular, in case that a terminal attempts to transmit data to a neighbor terminal by itself, the data needs to be transmitted to a relay node, which makes the terminal burdensome. Thus, although power consumption is adjustable to some extend using a relay node, it is still burdensome to utilize the relay node for a terminal-to-terminal communication absolutely.
FIG. 2 is a diagram for a terminal-to-terminal communication using a relay node.
Referring to FIG. 2, assume that a terminal A transmits data to a relay node and that the relay node transmits the data to a terminal B in the same relay node area via local forwarding. In this case, a terminal should match a power level to overcome a pathloss to a relay node and the relay node should transmit data with power to overcome a pathloss to a terminal. If a power level is raised, interference with another neighbor terminal increases. Hence, a frequency resource used for the data transmission is hardly usably by another terminal.
Formula 1 is a pathloss model used by Bluetooth. Referring to Formula 1, a pathloss exponentially increases in proportion to a distance.Pathloss=20 log(4*pi*r/lambda) for r≦8m Pathloss=58.3+33 log(r/8) for r>8  [Formula 1]
FIG. 3 is a graph of a pathloss model of Bluetooth.
For instance, assume that a distance between terminals is 2 meters and that a distance between a terminal and a rely node is 20 meters, when a communication is performed via a relay node, transmission power required for the consumption of the terminal is greater than that of a direct terminal-to-terminal communication by 25 dB or higher. In particular, it can be observed that a difference in an amount of energy is about 300 times or more. Thus, in order to perform a transmission via a relay node, a terminal should consume a power considerably more than that required for a direct communication to result in a very inefficient energy consumption pattern. Such a result may generate a serious difference from such a battery-based operating terminal as an M2M device having difficulty in battery replacement.
In order to enable a short-range communication with low power consumption, a terminal should have a structure of utilizing a different radio access technology (hereinafter abbreviated RAT). However, if a terminal includes multi-RAT, a chip size increases. And, the terminal should have a multi radio frequency (hereinafter abbreviated RF) structure and an antenna structure, whereby complexity increases as well.